ethernet_slirp: Add libslirp EthernetConnection backend

The existing Ethernet connection implementation is PCAP-based. This
allows bridging your virtual machine directly to the host's network, but
requires elevated permissions on your OS and appropriate drivers which
often aren't available on Wi-Fi cards.

This commit adds a libslirp-based backend that provides just enough
internal networking to handle TCP/IP out of the guest to host, LAN or
Internet. As such things like ICMP, IPX and NetBIOS don't work properly.

The use case for this connection is to give Internet access to the guest
for things like IRC, HTTP and other connections to online services.

configure.ac

@@ -430,19 +430,12 @@ LDFLAGS="$LDFLAGS -L$pwd/vs2015/sdlnet/linux-host/lib"
 CPPFLAGS="$CPPFLAGS -I$pwd -I$pwd/vs2015/sdlnet/linux-host/include -I$pwd/vs2015/sdlnet/linux-host/include/SDL"
 CXXFLAGS="$CXXFLAGS -I$pwd -I$pwd/vs2015/sdlnet/linux-host/include -I$pwd/vs2015/sdlnet/linux-host/include/SDL"
 
-dnl LIBRARY TEST: SDLnet initial test
-AC_CHECK_HEADER(SDL_net.h,have_sdl_net_h=yes,)
-
 if test x$enable_emscripten != xyes; then
 dnl Some target detection and actions for them
 case "$host" in
     *-*-cygwin* | *-*-mingw32*)
        LIBS="$LIBS -lwinmm -ldsound -limm32 -lole32 -loleaut32 -lversion -lsetupapi"
        CXXFLAGS="$CXXFLAGS -mno-ms-bitfields"
-       if test x$have_sdl_net_h = xyes ; then
-         dnl HACK: We have to put SDL_net THEN winsock libraries or else the linker can't resolve things properly
-         LIBS="$LIBS -lSDL_net -lwsock32 -lws2_32 -lwinspool -lshlwapi -liphlpapi"
-       fi
        dnl FEATURE: Whether to support direct parallel port passthrough
        AC_DEFINE(C_DIRECTLPT, 1, [ Define to 1 if you want parallel passthrough support (Win32, Linux).])
        dnl FEATURE: Whether to support direct serial port passthrough
@@ -930,11 +923,13 @@ if test -n "$SDL2_LIBS"; then
   AC_DEFINE(C_SDL_NET,1)
   AC_DEFINE(C_MODEM,1)
   AC_DEFINE(C_IPX,1)
+  have_sdl_net=yes
  elif test x$have_sdl_net_lib = xyes -a x$have_sdl_net_h = xyes ; then
   LIBS="$LIBS -lSDL_net"
   AC_DEFINE(C_SDL_NET,1)
   AC_DEFINE(C_MODEM,1)
   AC_DEFINE(C_IPX,1)
+  have_sdl_net=yes
  else
   AC_MSG_WARN([Can't find SDL_net, internal modem and ipx disabled])
  fi
@@ -944,6 +939,7 @@ else
   AC_DEFINE(C_SDL_NET,1)
   AC_DEFINE(C_MODEM,1)
   AC_DEFINE(C_IPX,1)
+  have_sdl_net=yes
  else 
   AC_MSG_WARN([Can't find SDL_net, internal modem and ipx disabled])
  fi
@@ -992,6 +988,24 @@ case "$host" in
     ;;
 esac
 
+dnl FEATURE: Whether to use libslirp, and enable userspace TCP/IP emulation
+AH_TEMPLATE(C_SLIRP, [Define to 1 to enable userspace TCP/IP emulation, requires libslirp])
+PKG_CHECK_MODULES(libslirp, slirp, [
+		have_slirp=yes
+		AC_DEFINE(C_SLIRP,1)
+		LIBS="$LIBS ${libslirp_LIBS}"
+		CFLAGS="$CFLAGS ${libslirp_CFLAGS}"
+		CPPFLAGS="$CPPFLAGS ${libslirp_CFLAGS}"], [
+		have_slirp=no
+		AC_MSG_WARN([Can't find libslirp, userspace TCP/IP emulation disabled])])
+case "$host" in
+    *-*-cygwin* | *-*-mingw32*)
+       if test x$have_slirp = xyes; then
+         LIBS="$LIBS -lintl -liconv"
+       fi
+    ;;
+esac
+
 if test x$enable_x11 != xno; then
   dnl FEATURE: Whether to use X11 XKBlib
   AH_TEMPLATE(C_X11_XKB,[define to 1 if you have XKBlib.h and X11 lib])
@@ -1137,6 +1151,18 @@ fi
 dnl placeholder
 AH_TEMPLATE(C_ICONV_WIN32,[Define to 1 to use Win32 functions in iconv backend])
 
+dnl If networking is used, add networking libraries last on Windows
+dnl so SDL_net, fluidsynth and libslirp successfully link to them
+if test x$enable_emscripten != xyes; then
+case "$host" in
+    *-*-cygwin* | *-*-mingw32*)
+       if test x$have_sdl_net = xyes -o x$enable_libfluidsynth = xyes -o x$have_slirp = xyes; then
+         LIBS="$LIBS -lwsock32 -lws2_32 -lwinspool -lshlwapi -liphlpapi"
+       fi
+    ;;
+esac
+fi
+
 AC_CONFIG_FILES([ 
 Makefile
 src/Makefile
@@ -1181,4 +1207,3 @@ chmod +x make-rpm.sh
 
 # HACK: Write all PACKAGE defines to vs2015/config_package.h so Windows builds are kept in sync
 grep -E 'PACKAGE|VERSION' config.h > $srcdir/vs2015/config_package.h
-